sep_by, and some small tweaks

1 files changed, 13 insertions, 3 deletions

diff --git a/src/parcom/parser.cr b/src/parcom/parser.cr
index d9919e1..d91ac84 100644
--- a/src/parcom/parser.cr
+++ b/src/parcom/parser.cr
@@ -255,7 +255,7 @@ module Parcom
     end
 
     # Same as `#+`, but discards the second parser's result.
-    def <<(p2 : Parser(T, V)) : Parser(T, U) forall V
+    def <<(p2 : Parser(T, _)) : Parser(T, U)
       p1 = self
       (p1 + p2).map(&.first).named("#{p1.name} << #{p2.name}")
     end
@@ -267,7 +267,7 @@ module Parcom
     end
 
     # Creates a new parser from `self` that parses with `self` as many times
-    # as possible. If it never succeeds, returns an empty list.
+    # as possible. Returns an empty list if it never succeeds.
     def many : Parser(T, Array(U))
       p = self
       Parser(T, Array(U)).new("Many: #{p.name}") do |tokens|
@@ -371,7 +371,6 @@ module Parcom
       min = n < m ? n : m
       extra = (n - m).abs
 
-      p = self
       (exactly(min) + at_most(extra)).map do |t|
         t.first + t.last
       end.named("Between #{min}-#{extra} <#{@name}>")
@@ -412,6 +411,17 @@ module Parcom
 
       at_least(r.begin).named("Between #{r.begin}-infinity <#{@name}> (from inifnite range)")
     end
+
+    # Creates a new parser from `self` and another parser that tries to
+    # parse with `self` repeatedly, alternating with `sep` and discarding
+    # the results of `sep`. This parser will fail if it cannot parse the first
+    # instance of `self`. It will succeed if it can parse an instance of `self`
+    # that is not followed by any `sep` instances.
+    def sep_by(sep : Parser(T, _)) : Parser(T, Array(U))
+      (self + (sep >> self).many).map do |tup|
+        tup.last.unshift(tup.first)
+      end.named("<#{@name}> sep by <#{sep.name}>")
+    end
   end
 end